Yes I agree with you, if you are using your Discretionary income to accumulate Bitcoin you won't have much issues with your invest, however when using your Discretionary income to invest in Bitcoin you also need to have an emergency funds because it is it that safe you totally, using your discretionary income alone cannot save you from financial issues as a bitcoin investor you need a very strong backup funds while using only your Discretionary income to accumulate Bitcoin.
DCA strategy of accumulation is a very good investment strategy now the reason why it is a very good investment strategy is because it gives you the opportunity to accumulate bitcoin at every single price that comes up meaning you don't have to wait for a particular price for you to start accumulating and it gives you fast achievement.
You need to understand that it is after you have taken out the emergency funds and all other necessary expenses have been covered, that is when you have discretionary income left so there is no reason to be concerned again about emergency funds after having discretionary income because it should have been handle before you call any remaining funds a discretionary income.
Overall the best form of purchasing bitcoin is indeed DCA at any time interval that suits your income best is having a steady stream of income then you can simply set a time line pending on when the income arrives and then steadily use the duration, if after a long period of time accumulating for emergency funds you have maybe held enough (although there isn’t enough for emergency ) you can then aggressively accumulate more by channeling some of this to investing into bitcoin with the discretionary income